The present invention relates to an amplitude modulator comprising a differential pair of transistors called differential input pair receiving a modulating input signal, and a Gilbert cell multiplying a signal called carrier signal by the modulating input signal.
The Gilbert cell, which forms the essential part of such a modulator, is known from the document "A Precise Four-Quadrant Multiplier with Subnanosecond Response" by Barrie Gilbert, published in IEEE Journal of Solid State Circuits, Vol. SC-3, No. 4, December 1968.
The circuit presented by Barrie Gilbert, commonly used in amplitude modulators, performs a multiplication of a modulating input signal of the form Vm(t)=K1. sin .omega..sub.m t by a carrier signal of the form Vc(t)=K2. sin .omega..sub.c t. The differential pair forming the lower stage of the Gilbert cell receives the carrier signal Vc, whereas the differential pairs forming the upper stage of the Gilbert cell receive the modulating input signal after it has been amplified in the differential input pair, to which a DC component called input offset voltage is added which may be realized in various ways, via a supply voltage divider bridge circuit, or, for example, on the basis of a regulated voltage. This input offset voltage performs two essential functions: it permits the amplitude modulator of producing an output signal under any circumstances, even if there is no modulating input signal or if the latter has a zero value. On the other hand, this input offset voltage permits of the production of a signal Vout(t) on the output, which signal contains a component that is proportional to the single carrier signal. The output signal Vout thus has the form of
Vout(t)=A.(1+K. sin .omega..sub.m t). sin .omega..sub.c t, PA1 a unit, called entry unit, which receives the data and produces signals of two different kinds, one called audio signal, the other called video signal, PA1 an audio signal processing module featuring one input and one output, which module includes and oscillator which provides a frequency-turnable output signal, PA1 a video signal processing module featuring one input and one output, PA1 an adder featuring two inputs and one output, which adder performs a superposition of the signals which are available at the outputs of the audio and video signal processing modules. PA1 a mixer enabling the tuning to a nominal frequency of the frequency of the signal which is available at the output of the adder, PA1 characterized in that the audio signal processing module comprises an amplitude modulator as described above, amplitude modulator for which the modulating signal is constituted by the audio signal produced by the entry unit, amplitude modulator for which the carrier signal is constituted by the output signal of the oscillator, amplitude modulator whose output signal constitutes the output of the audio signal processing module.
which indeed corresponds to the equation of an amplitude modulated signal.
The input offset voltage itself presents a major drawback. Its value is to be of the same order of magnitude as the amplitude with which the modulating input voltage is varied. Depending on the case, its value may be as little as several millivolts, its value being thus hard to check precisely and in a stable manner, which is highly annoying.
It is an object of the present invention to remedy this drawback by proposing an amplitude modulator in which the input offset voltage is no longer necessary for ensuring the existence of the output signal if there is no input signal, nor for generating on the output a signal whose equation corresponds to that of a modulated signal described above.